A magnetic random access memory (MRAM) wafer is generally a silicon wafer onto which is built, or disposed, a group of magnetic memory chips (also known as dies). A typical MRAM wafer might have more than 10000 magnetic memory chips. The magnetic memory chips, or memory chips, are disposed across the MRAM wafer in a grid pattern prior to separation from the MRAM wafer for subsequent use in electronic devices.
Each memory chip is comprised of one or more memory arrays, with each memory array comprised of a plurality of bit cells (also referred to as “memory cells”). The bit cells of a memory array store retrievable data. The data in each bit cell is stored as an orientation of magnetization in the magnetic layers of the bit cell. For example, in one type of bit cell known as a magnetic tunnel junction, the bit cell consists of two magnetic layers separated by a thin insulating tunnel barrier. The logic value of the bit cell is determined by the relative orientations of the magnetizations of the two layers. If the orientations of magnetization of the layers point in the same direction (referred to as a “parallel” orientation), the resistance of the bit cell is low, and a logic value 0 is assigned. If the orientations of magnetization of the layers point in opposite directions (referred to as an “anti-parallel” orientation), the resistance of the bit cell is high and a logic value 1 is assigned. Typically, the orientation of magnetization of one layer is fixed or pinned (the “reference layer”), whereas the orientation of magnetization of the other layer can be varied by an applied magnetic field (the “data layer” or “sense layer”).
Conductive traces, referred to as word lines and bit lines, are routed across the bit cells that form the memory arrays. Word lines extend along rows of the bit cells and bit lines extend along columns of the bit cells. Selected word lines and bit lines are energized with electric currents in combination to create magnetic fields that switch the orientation of magnetization of a selected bit cell from parallel to anti-parallel, or vise versa. The word lines and bit lines may be collectively referred to as write lines. The write lines can also be employed to read the logic value stored in the bit cell. Alternatively, separate sense lines can be added for purposes of reading stored data from the bit cells.
The magnetization stored in a bit cell creates magnetic field lines analogous to those associated with magnets generally, and as such, the magnetization has positive magnetic poles and negative magnetic poles, or charges, which serve as source and sink for the field lines. The closer the positive magnetic poles are to the negative magnetic poles, the stronger the magnetic field created by those poles.
High-density memory is characterized by the tight packing of small bit cells within memory arrays. Small bit cells have magnetic poles that are inherently close together and that produce strong magnetic fields. Additionally, the magnetic field associated with positive and negative poles within the bit cell is referred to as a demagnetization field, since it opposes the magnetization direction within the bit cell. Demagnetization fields become stronger as the magnetic poles that give rise to them are brought closer together.
To maintain the desired magnetization orientation in the data layer (0 and 1 state), sufficient magnetic anisotropy must be present in the magnetic film to overcome the demagnetization field. Two predominant magnetic anisotropy terms are shape anisotropy and magnetocrystalline anisotropy, with shape anisotropy being predominant for sub-micrometer bit cells. Reduction of the demagnetization field along the easy axis of the bit cells would enhance the stability of the bit cells. More stable magnetic bit cells are clearly desirable.
Coercivity is a measure of a material's resistance to magnetization reversal by an applied magnetic field. In an unpatterned film of soft, ferromagnetic material, the coercivity is typically determined by the resistance to domain wall motion. Such coercivity can be substantially lower than the magnetic anisotropy of the film. However, thin magnetic films patterned to sub-micrometer dimensions often do not contain domain walls. Coercivities in such patterned films are generally comparable to the total magnetic anisotropy of the bit cell, which is dominated by shape anisotropy. For bit cells with an aspect ratio greater than two, shape anisotropy, and hence coercivity, is inversely proportional to the width of the bit cell. As MRAM bit cells shrink in size to accommodate higher bit cell densities, the coercivity climbs, and with it the amount of electric current needed to write the bit cells.
For small magnetic particles the magnetization switching mechanism is often governed by magnetization rotation rather than domain wall motion. As the magnetization rotates through the hard axis (short dimension) of the bit cell, the demagnetization field reaches a maximum value. In order to switch the magnetization orientation, the applied magnetic field must overcome the hard axis demagnetization field. Reduction of this demagnetization field would lower the coercivity of the bit cell, and hence, the current necessary to write the bit cell. Lower write currents in MRAM devices are desirable.
High-density memory, which is useful for advanced computing applications, is associated with closely packed, small bit cells. Demagnetization fields in small bit cells cause instability of the easy axis magnetization orientation and high switching fields.